“Composite materials” constitute one of the categories of advanced materials and defined as materials composed of two or more distinctly identifiable constituents. In composite materials, one material serves as a matrix, whereas another material serves as a reinforcement. Composites are generally more mechanically strong, light weight, corrosion resistant and dimensionally stable in comparison to their conventional counterparts, and used in a growing number of industries.
Fiber reinforced thermoset polymer composites are widely employed composite materials for research and development in structural engineering, predominantly in the high profile structural engineering industries such as aviation, military, space shuttle manufacturing and the like. Unidirectional fabric reinforced thermoset composites are examples of thermoset composites finding extensive load bearing applications in virtually all sectors of military engineering such as military bridging, naval structures, light weight combat aircrafts and the like.
Manufacturing of large and thick composite structures for structural applications is carried out by processes such as Vacuum Assisted Resin Transfer Molding (VARTM). In this process, fabric is stacked on the mould, sealed with vacuum bag and the resin is driven into the system by vacuum. In recent years this method has widely been used to develop load bearing fiber reinforced polymer composites.
The polymer resin being driven by vacuum undergoes a global flow from the resin source towards the vacuum line. Therefore, the processability of the resin is a critical factor as far as the infusion through thicker layers of fabric, under the application of vacuum is concerned. While fabricating large and thick composite structures by the VARTM process, there are certain issues to be taken care of such as application of proper magnitude of vacuum, ensuring zero leakage, proper positioning of vacuum lines and resin lines, and the distribution of resin flow and permeability.
Apart from foregoing essential requirements, certain stringent characteristics of the polymer resin are also required for this infusion technique. The polymer system as employed in the preparation of reinforced polymer composites serves two primary functions (a) to distribute the stress between fibers for structural efficiency and (b) to rigidize the structure to provide mechanical utility. It also determines many secondary functions such as electrical resistance and fire resistance which contribute to the ultimate utility of product. Viscosity of the liquid resin and its rate of conversion to a solid mass especially during the fabrication of large and thick composite structures through the VARTM process are also relevant from a processing point of view. The exothermic nature of the epoxy's cure reaction further presents challenges with regard to the control of the whole process carried out on a large scale. Apart from the viscosity and other characteristic features, the VARTM process also demands ideal gelation conditions for the resin.
Structural composites developed for military applications use unidirectional glass or carbon fabric. Other than the viscosity and the reactivity profile of the resin formulation, permeability of the reinforcing fabric and distribution of resin through it (which ultimately determines the extent of resin saturation) also determines the infusion temperature, infusion time, holding time, and vacuum applications. Low permeability of unidirectional fabric causes difficult infusion of the polymer resin mix through the fabric.
Existing Knowledge:
A commercially available resin formulation that is used in the manufacture of large and thick composite structures is Diglycidyl Ether of Bisphenol A (DGEBA) based epoxy resin (Araldite LY1564) with an amine hardener (Aradur 3486) from Huntsman ltd. At a proportion of 100:34, this resin-hardener mixture has an initial viscosity of 200 cps at 25° C. Cross-linking of the aliphatic amine hardener with epoxy resin at faster rate tremendously increases the viscosity of the system. The increased viscosity makes infusion of thick and large structures difficult and in worst case leads to cure loss and heavy financial non-recoverable loss. This also leads to formation of dry patches in composite structures, which might cause a catastrophic failure under load. Further, the cross-linking of DGEBA based epoxy resin and amine hardener produces substantial exotherm which contributes to the threshold to provide exponential hike in viscosity in a short duration.
Considering the above described hurdles encountered during processing of a resin formulation while preparing a composite structure for high end structural engineering applications, an improved matrix formulation with appropriate processing characteristics as an alternative to the commercially available ones is highly desirable.
Objects:
Some of the objects of the present disclosure are described herein below:
It is an object of the present disclosure to ameliorate one or more problems of the prior art or to at least provide a useful alternative.
Another object of the present disclosure is to provide a thermoset resin formulation for the fabrication of large and thick composite structures.
Still another object of the present disclosure is to provide a thermoset resin formulation with improved/optimized processing characteristics that include a low viscosity and a tuned reactivity profile.
Yet another object of the present disclosure is to provide a thermoset resin formulation having ideal gelation conditions.
Further object of the present disclosure is to provide a thermoset resin formulation wherein curing/post curing of the resin formulation is feasible within a stipulated period of time to result in a high glass transition temperature.
Still a further object of the present disclosure is to provide a thermoset resin formulation wherein preparation of large and thick composite structures employing the resin formulation is carried out under thermally sustained conditions.
A further object of the present invention is to provide large and thick composite structures with excellent mechanical properties.
Other objects and advantages of the present invention will be more apparent from the following description when read in conjunction with the accompanying figures, which are not intended to limit the scope of the present invention.